Oh, another point.  If you reduce the size of the Blucher to 8000 tons you will only need an 8000 ton jumpship instead of 11000 ton. It's not a necessary step, but might be logistically convenient.

It's also good role-playing to heavily defend the home system... even if it isn't really under much of a threat.

Asteroid PDCs are great for defending jump point. Their only real weakness is the tendency of the enemy to run out of their range, but the fighters plus a 500km sensor should give you plenty of buffer.  The PDC should be effective. My main concern would be that 70 mkm is within reasonable retaliation range for enemy missile ships.   You may have incoming salvos before the fighters reach strike distance. All that armor and CIWS means that probably wont be a problem, but it's never good to underestimate the AI.

One possibility is to build a special sensor-only base, and put it farther away from the gate - if only that sensor is active, the enemy may not even see the fighter PDC. It also gives you redundancy.

The cost still makes me wince... How important is this jump point? You're looking at a 7500+ credit investment, not counting the missiles.   Hardly chump change - that's more than a cruiser group in my current game.

I came up with a new idea.

I found a Jump Point (and build a Jump Gate on it). This Jump Point is only 70,470,000 Km from an Asteroid away (I disabled asteroid orbit due performance issues). So I designed a PDC for defending this Jump Gate if anything hostile comes through. Make this any sense or is it useless to use an PDC for defending Jump Gates? Here the Design:

Hessen class Fighter Base    44,800 tons     1077 Crew     5277.5 BP      TCS 896  TH 0  EM 0
Armour 14-112     Sensors 1/588     Damage Control Rating 0     PPV 0
Hangar Deck Capacity 12000 tons     Magazine 3080   

Fuel Capacity 1,000,000 Litres    Range N/A
CIWS-120 (30x6)    Range 1000 km     TS: 12000 km/s     ROF 5       Base 50% To Hit
Size 4 Anti-ship Missile (770)  Speed: 32,000 km/s   End: 15.6m    Range: 30m km   WH: 5    Size: 4    TH: 277 / 166 / 83

Active Search Sensor 3000 t / 501 M Km (1)     GPS 35280     Range 501.0m km    Resolution 60

Strike Group
44x Fighter Fighter   Speed: 11600 km/s    Size: 5
4x AWACS Fighter Recon Fighter   Speed: 12083 km/s    Size: 4.8

Missile to hit chances are vs targets moving at 3000 km/s, 5000 km/s and 10,000 km/s


This design is classed as a Planetary Defence Centre and can be pre-fabricated in 18 sections

The Base has 4 Strikeforces of 11 Fighter an 1 Awacs fighter for each strikeforce
And here the two fighter:

Fighter class Fighter    250 tons     4 Crew     51.3 BP      TCS 5  TH 29  EM 0
11600 km/s     Armour 1-3     Shields 0-0     Sensors 1/1/0/0     Damage Control Rating 0     PPV 2.4
Maint Life 10.82 Years     MSP 13    AFR 5%    IFR 0.1%    1YR 0    5YR 3    Max Repair 17 MSP
Magazine 16   

Fighter Magneto-plasma Drive (1)    Power 57.6    Fuel Use 8400%    Signature 28.8    Armour 0    Exp 80%
Fuel Capacity 10,000 Litres    Range 0.9 billion km   (20 hours at full power)

Size 4 Fighter Launcher (4)    Missile Size 4    Hangar Reload 30 minutes    MF Reload 5 hours
Missile Fire Control 3000t / 42M Km (1)     Range 42.9m km    Resolution 60
Size 4 Anti-ship Missile (4)  Speed: 32,000 km/s   End: 15.6m    Range: 30m km   WH: 5    Size: 4    TH: 277 / 166 / 83

Missile to hit chances are vs targets moving at 3000 km/s, 5000 km/s and 10,000 km/s

This design is classed as a Fighter for production, combat and maintenance purposes

The Recon Fighter is part of the strikeforce in case (by any ridicolous reason) that the PDC goes down. So he is only back  up for the strikeforce

AWACS Fighter class Recon Fighter    240 tons     7 Crew     87.7 BP      TCS 4.8  TH 29  EM 0
12083 km/s     Armour 1-3     Shields 0-0     Sensors 1/1/0/0     Damage Control Rating 0     PPV 0
Maint Life 5.78 Years     MSP 23    AFR 4%    IFR 0.1%    1YR 1    5YR 18    Max Repair 63 MSP

Fighter Magneto-plasma Drive (1)    Power 57.6    Fuel Use 8400%    Signature 28.8    Armour 0    Exp 80%
Fuel Capacity 10,000 Litres    Range 0.9 billion km   (20 hours at full power)

AWACS Fighter Active Search Sensor 3000t / 53M Km (1)     GPS 3780     Range 53.7m km    Resolution 60

This design is classed as a Fighter for production, combat and maintenance purposes

Am I completely wrong or is it a viable idea for defending Systems?

EDIT: perhaps a better way of putting it is that there is little point in carrying a SMALL Thermal array. Either carry a big one, or none at all.

Anything over size 1 is a military system. If you are putting them on freighters/colony ships, or civilian geo ships, you can't have the big ones.

The Thermal sensors are also likely a waste of space... they have a range of about 50 m-km against a typical Light Cruisers's thermal signature. What could they see that wouldn't already be spotted on RADAR? The EM sensor on the other hand is useful, for picking up enemy active sensors at a much greater range.

I do not understand the thing with passiv sensors.  May I be a bit dorf but I dont understand the mechanic behind that.  I only use them for the case they are vital and I simply don`t know it

First up, before I forget; you seem to have two of every sensor. This is really a waste of tonnage, because if the enemy has already taken out one, the ship is in dire straits already, and multiple sensors on the same ship has no other benefits.

Another thought is that sensor ships are the eyes of your fleet - vital to you and a target for the enemy. My sensor ships generally have a few extra layers of armor compared to combat ships.

-

The way passive sensors work is that their range is displayed against a signature level of 1000. So your sensors will detect a strength 1000 thermal signature at 44m km; larger signatures will be detected further away and smaller signatures closer to you. (I think the increase and decrease is linear, since they don't have anything like the resolution of active sensors).

Thermal passives detect engine emissions - so anything moving (or idling with the engines on) will show up. EM passives detect enemy active sensors and shields - which usually don't come on until they've detected something via another means.

So really the problem here is that your thermal's range is a lot less than the range of your active sensor's (179m km vs 44m km against roughly 'average' targets). What blue emu suggested - ditching thermals altogether - is a matter of doctrine. If you intend to run around with active sensors up all the time, then the thermals are pretty much useless. However, active sensors are a big blinking 'HERE I AM' sign, so if there's anything around it will probably see you. Thermals (particularly larger thermals) will allow you to detect enemies before they see you - although you'll still need to switch on your own actives to target them.

I revised the Design and it looks like this now.

Bismarck Detector class Surveillance Cruiser    7,950 tons     722 Crew     2794.6 BP      TCS 159  TH 800  EM 0
5031 km/s     Armour 5-35     Shields 0-0     Sensors 1/44/0/0     Damage Control Rating 10     PPV 0
Maint Life 5.39 Years     MSP 2197    AFR 50%    IFR 0.7%    1YR 127    5YR 1898    Max Repair 630 MSP

Military Magneto-plasma Drive (10)    Power 80    Fuel Use 60%    Signature 80    Armour 0    Exp 5%
Fuel Capacity 200,000 Litres    Range 75.5 billion km   (173 days at full power)

Active Search Sensor MR178-R60 (50%) (2)     GPS 12600     Range 178.9m km    Resolution 60
Active Search Sensor MR46-R1 (50%) (2)     GPS 420     Range 46.2m km    Resolution 1
EM Detection Sensor EM4-44 (50%) (2)     Sensitivity 44     Detect Sig Strength 1000:  44m km

This design is classed as a Military Vessel for maintenance purposes

Blucher Missile Cruiser class Cruiser    11,000 tons     973 Crew     1654.65 BP      TCS 220  TH 1120  EM 0
5090 km/s     Armour 5-44     Shields 0-0     Sensors 1/1/0/0     Damage Control Rating 7     PPV 40
Maint Life 4.82 Years     MSP 658    AFR 138%    IFR 1.9%    1YR 47    5YR 701    Max Repair 42 MSP
Magazine 964   

Military Magneto-plasma Drive (14)    Power 80    Fuel Use 60%    Signature 80    Armour 0    Exp 5%
Fuel Capacity 300,000 Litres    Range 81.8 billion km   (186 days at full power)

Size 4 Missile Launcher (10)    Missile Size 4    Rate of Fire 30
Missile Fire Control 3000t / 107M Km (2)     Range 107.3m km    Resolution 60
Size 4 Anti-ship Missile (241)  Speed: 30,000 km/s   End: 41.7m    Range: 75m km   WH: 5    Size: 4    TH: 180 / 108 / 54

Missile to hit chances are vs targets moving at 3000 km/s, 5000 km/s and 10,000 km/s

This design is classed as a Military Vessel for maintenance purposes
Scharnhorst Escort class Escort Cruiser    8,000 tons     632 Crew     1670.5 BP      TCS 160  TH 800  EM 0
5000 km/s     Armour 5-35     Shields 0-0     Sensors 1/1/0/0     Damage Control Rating 9     PPV 10
Maint Life 4.7 Years     MSP 1175    AFR 56%    IFR 0.8%    1YR 87    5YR 1306    Max Repair 315 MSP
Magazine 626   

Military Magneto-plasma Drive (10)    Power 80    Fuel Use 60%    Signature 80    Armour 0    Exp 5%
Fuel Capacity 200,000 Litres    Range 75.0 billion km   (173 days at full power)

Size 1 Missile Launcher (10)    Missile Size 1    Rate of Fire 10
Anti Missile Fire Control 7.5M Km (2)     Range 69.3m km    Resolution 1
Size 1 Anti-missile Missile (626)  Speed: 36,000 km/s   End: 1.4m    Range: 3m km   WH: 1    Size: 1    TH: 384 / 230 / 115

Missile to hit chances are vs targets moving at 3000 km/s, 5000 km/s and 10,000 km/s

This design is classed as a Military Vessel for maintenance purposes

A few notes:
I got a bit better at Missile Technology
Invented better Armor so I have now 5 Layers of it instead of 3
I have not invented new Sensors so 50% resistent still in effect

Agreed... you are carrying at least twice (and maybe four times) more missiles than necessary, which requires more engines, which makes your ships heavier, requiring still more engines, which increases breakdown frequency, requiring more engineering sections, which makes your ships heavier, requiring still more engines...

Reducing your magazine capacity by a factor of two or three (or even more) will allow you to use fewer engines and engineering sections, resulting in much smaller ships. This applies to both the Scharnhorst and Blucher designs.

Other comments:

Scharnhorst: Your AMMs are unnecessarily long-ranged. Half that range would be fine, which allows you to use half of the tonnage now devoted to fuel to boost the missiles engine and agility instead, giving an increased to-Hit chance. It also would allow you to carry Missile PD-FCs only half as massive (ie: half the range).

Blucher: I would trade a couple of missile tubes for a second fire control. Penetrating enemy PD is more a function of the number of salvos launched rather than the raw number of missiles. Each enemy PD FC can only target a single salvo of missiles in each increment, so more salvos (rather than more missiles) increases the chance of some slipping through. Four missiles per salvo is a good number, given the prediliction of the AI for firing AMMs in clusters of three.

Bismarck: Not sure I see any advantage in hardening your electronics (50%)... Microwaves are extremely short-ranged... even shorter than Lasers... and by the time the opponent is close enough to hit you with microwaves, you're probably dead anyway.

The Thermal sensors are also likely a waste of space... they have a range of about 50 m-km against a typical Light Cruisers's thermal signature. What could they see that wouldn't already be spotted on RADAR? The EM sensor on the other hand is useful, for picking up enemy active sensors at a much greater range.